1. Field of the Invention
Thermoplastic polymer moulding materials, which may be based, for example, on polymethyl methacrylate (PMMA), are used for a wide variety of different applications. For this purpose, the materials are extruded or injection-moulded to give mouldings.
2. Description of Background Art
Mouldings are nowadays used widely for production of parts subject to high stress, for example movable parts (automobile interiors and exteriors, covers for electronic devices such as covers for mobile phones, computers, organizers, MP3 players or televisions), opaquely coloured installable components (for example in the automotive industry: exterior mirrors, pillar trim, mirror triangles), or opaquely coloured everyday items. Owing to high stress, the surface of the mouldings thus used tends to form scratches, which in many cases are unacceptable for visual reasons. In this context, mouldings which have been produced by means of injection moulding are particularly scratch-sensitive. Furthermore, further properties of the mouldings can be altered by a modification of the surface. For example, a moulding can be provided with a hydrophobic or hydrophilic coating in order, for example, to alter wettability with water or other liquids. In addition, the surface of a moulding can be configured so as to be reflective or to reduce reflection. Furthermore, these mouldings may also have soil-repellent or antibacterial properties, which are in many cases achieved by a modification of the surface.
To improve scratch resistance, to modify the hydrophobicity/hydrophilicity of the surface and the reflection properties, and to provide the surfaces with antimicrobial and/or soil-repellent properties, the mouldings detailed above can be provided with paint layers. However, the conventional application of reactive paints is relatively complex and therefore costly.
For this reason, there has already been development of processes by which a scratch-resistant layer can be applied to the mouldings relatively inexpensively by means of injection moulding processes. For example, publications JP 11300776 and JP 2005074896 describe injection moulding processes in which a moulding with a scratch-resistant layer is obtained.
Publication JP 11300776 (Dainippon Toryo, 1998) describes a two-stage RIM process. First, metathesis RIM of dicyclopentadiene gives a moulding. After hardening, the moving part of the RIM mould is retracted, so as to give rise to a defined gap between moulding and mould. In a second RIM process, a coating material is injected into this gap, consisting of acrylic-functionalized urethane oligomers, styrene, diacrylate crosslinkers and optionally fillers and pigments (TiO2, talc), and cured by free-radical means at 95° C. for 2 min.
Document JP 2005074896 (Toyota Motor Corp.; Dainippon Toryo Co.) likewise describes an RIM process. In a first, conventional injection moulding step, a polymer, especially polycarbonate (PC), is processed to give a flat moulding. The mould subsequently opens up to form a narrow gap, and a reactive solution composed of acrylate-functionalized urethane oligomers, acrylate crosslinkers, inhibitors and an organic peroxide initiator is injected within a few seconds and cured. At 95° C., curing is complete after a few seconds, and the composite is demoulded after 90 s. It has good scratch resistance, bond strength, and thermal cycling and warm water cycling stability. Obligatory in all cases is the presence of a urethane oligomer which is formed from isophorone diisocyanate or bis(isocyanocyclohexyl)methane units.
The mouldings detailed above already have good properties. However, the production is time-consuming, and so the process is costly overall. Premature polymerization of the reactive mixture in the injection moulding apparatus constitutes a further problem with the injection moulding process described in publications JP 11300776 and JP 2005074896, such that it is barely possible to achieve short cycle times by means of these processes in mass production.
Furthermore, problems occur in many cases with the service lives of the system, since the injection moulds are in many cases not sufficiently impervious to the reaction mixtures, such that they can come into contact with moving parts of the system.
The long cycle times detailed above can additionally lead to quality problems with the mouldings produced. In this regard, it should be noted that the moulding materials are subject to thermal stress in the injection moulding apparatus, which can lead to degradation of the polymers. This can alter, for example, the mechanical and optical properties of the moulding materials, for example the colour, and as a result also those of the mouldings.
For particular applications, it is advantageous when the surface is structured. The production of structured surfaces by means of injection moulding technology is known. In this context, in the injection embossing technique, a mould provided with the appropriate structure reproduces the structure in the polymeric carrier material. A disadvantage is the wear on the structures embossed in the polymer material during use, or during cleaning operations.
As described above, these structured mouldings can subsequently be provided, for example, with a scratch-resistant coating. However, the coating applied subsequently lead to loss of the structure.
In order to counteract this, the structure can be embossed into the scratch-resistant coating in a downstream embossing step.
These additional process steps considerably increase the manufacturing costs.
Furthermore, there is continuing effort to improve the scratch resistance and the weathering stability of the mouldings thus obtained. In addition, there is a need to specify a process for surface modification of mouldings, by means of which the surface properties of mouldings can be matched to a wide variety of different requirements. For instance, the process should especially enable the production of hydrophobic or hydrophilic coatings in order, for example, to alter wettability with water or other liquids. Furthermore, the process should enable reflective coating or a reduction in reflection of the surface. In addition, it should be possible to configure the moulding with soil-repellent or antibacterial properties.